Optical module and method for manufacturing the same

ABSTRACT

A printed circuit board includes a first transmission line provided on an insulating base, a first ground conductor, a notch portion that exposes a part of the first ground conductor, a conductor provided in the notch portion and electrically connected to the first ground conductor, and a first electrode exposed on a main surface of the insulating base facing a flexible board and electrically connected to the first transmission line. The flexible board includes a second transmission line provided on an insulating sheet, a second ground conductor, a second electrode exposed on a main surface of the insulating sheet facing the printed circuit board and connected to the second transmission line, and a third electrode exposed on the main surface of the insulating sheet and connected to the second ground conductor. The conductor and the third electrode are connected by solder.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese application JP2019-001667 filed on Jan. 9, 2019, which is hereby expresslyincorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to an optical module and a manufacturingmethod for an optical module.

BACKGROUND

Currently, most of the Internet and telephone networks are constructedby optical communication networks. Optical modules used as interfacesfor routers/switches and transmission devices that are opticalcommunication devices have an important role in converting electricalsignals into optical signals. In general, an optical module includes anoptical subassembly containing an optical element, a printed circuitboard mounted with an IC for processing a signal including a modulatedelectric signal, and a flexible printed board electrically connectedtherebetween.

An optical subassembly may include a ground conductor on the end surfaceof a transmission line so that one component is orthogonal to the signalwiring of the transmission line, and the upper surface of the groundconductor and the lower surface of a ground conductor disposed on thelower surface of the other component are connected by solder. With suchan optical subassembly, part of the high-frequency signal may besuppressed from being radiated into the air from the transmission line,and signal transmission characteristics in a high-frequency band areimproved.

SUMMARY

According to some possible implementations, an optical module mayinclude: a printed circuit board; and a flexible board. The printedcircuit board may include an insulating base, a first transmission lineprovided on the insulating base, a first ground conductor disposed inthe insulating base, a notch portion formed on a side surface of theinsulating base such that the first ground conductor is partiallyexposed, a conductor provided in the notch portion and electricallyconnected to the first ground conductor, and a first electrode exposedon a main surface of the insulating base facing the flexible board andelectrically connected to the first transmission line. The flexibleboard may include an insulating sheet including a plurality ofinsulating layers, a second transmission line provided on the insulatingsheet, a second ground conductor disposed in the insulating sheet, asecond electrode exposed on a main surface of the insulating sheetfacing the printed circuit board and connected to the secondtransmission line, and a third electrode exposed on the main surface ofthe insulating sheet facing the printed circuit board and connected tothe second ground conductor. The first electrode and the secondelectrode may be electrically connected, and the conductor and the thirdelectrode may be connected by solder.

According to some possible implementations, a method for manufacturingan optical module may include: preparing an in-process printed circuitboard including: an insulating base, a first transmission line providedon the insulating base, a first ground conductor disposed in theinsulating base, and a first electrode exposed on a main surface of theinsulating base facing the flexible board and electrically connected tothe first transmission line; forming a notch portion on a side surfaceof the insulating base such that the first ground conductor is partiallyexposed from the side surface; forming a conductor electricallyconnected to the first ground conductor at the notch portion; preparinga flexible board including: an insulating sheet including a plurality ofinsulating layers, a second transmission line provided on the insulatingsheet, a second ground conductor disposed in the insulating sheet, asecond electrode exposed on a main surface of the insulating sheetfacing the printed circuit board and connected to the secondtransmission line, and a third electrode exposed on the main surface ofthe insulating sheet facing the printed circuit board and connected tothe second ground conductor; disposing the flexible board and theprinted circuit board so that at least a part of the third electrodeoverlaps at least a part of a region surrounded by the conductor and theside surface when viewed from a direction orthogonal to the main surfaceof the insulating base; and applying solder from the conductor to theelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view of an optical module.

FIG. 2 is a perspective view showing a state before a printed circuitboard and a flexible board are connected.

FIG. 3 is a perspective view showing a state before the printed circuitboard and the flexible board are connected.

FIG. 4 is a perspective view showing a state in which the printedcircuit board and the flexible board are connected.

FIG. 5 is a perspective view showing a state in which the printedcircuit board and the flexible board are connected.

FIG. 6 is a graph obtained by calculating transmission characteristicsfrom the printed circuit board to the flexible board by using a highfrequency structure simulator (HFSS) as a three-dimensionalelectromagnetic field simulator, according to an example and acomparative example.

FIG. 7 is a schematic view showing a cross-sectional structure of afirst transmission line and a center line of a second transmission line.

FIG. 8 is a schematic perspective view showing a cross-sectionalstructure of the first transmission line and the center line of thesecond transmission line.

FIG. 9 is a schematic plan view of the printed circuit board.

FIG. 10 is a schematic plan view of a printed circuit board according toanother example.

FIG. 11 is a schematic plan view of a printed circuit board according toanother example.

FIG. 12 is a graph obtained by calculating transmission characteristicsfrom the printed circuit board to the flexible board by using a highfrequency structure simulator (HFSS) as the three-dimensionalelectromagnetic field simulator.

FIG. 13 is a schematic plan view in which an overlapping portion of anotch portion and a third electrode is enlarged.

DETAILED DESCRIPTION

The following detailed description of example implementations refers tothe accompanying drawings. The same reference numbers in differentdrawings may identify the same or similar elements.

FIG. 1 is an external view of an optical module 1 for opticalcommunication in the present implementation. A modulated electric signalis transmitted to an optical subassembly 100 via a flexible board 300connected to a printed circuit board 200 by solder or the like from adriving IC (not shown) mounted on the printed circuit board 200. Theoptical subassembly 100 contains an optical element and includes aninterface that transmits and receives outgoing light or incident light.The optical subassembly 100 includes an eyelet 120 and an opticalreceptacle 2. Although not shown, the optical subassembly 100, theprinted circuit board 200, and the flexible board 300 are built in ahousing made of metal or the like to constitute the optical module 1.

Examples of the optical subassembly 100 include a transmitter opticalsubassembly (TOSA) that has a light-emitting element such as a laserdiode therein, converts an electrical signal into an optical signal, andtransmits the optical signal, a receiver optical subassembly (ROSA) thathas a light-receiving element such as a photodiode therein and convertsthe received optical signal into an electrical signal, a bidirectionaloptical subassembly (BOSA) that has both of these functions, and thelike. The present implementation can be applied to any of the aboveoptical subassemblies, and in the present implementation, a transmitteroptical subassembly will be described as an example.

FIG. 2 is a perspective view showing a state before the printed circuitboard 200 and the flexible board 300 according to the presentimplementation are connected, when viewed from above. FIG. 3 is aperspective view showing a state before the printed circuit board 200and the flexible board 300 according to the present implementation areconnected, when viewed from below.

As shown in FIGS. 2 and 3, the printed circuit board 200 includes aninsulating base 210, a first transmission line 220, a first groundconductor 230, a notch portion 240, a conductor 250, and a firstelectrode 260.

The first transmission line 220 that transmits an electrical signal toan optical element provided in the optical subassembly 100 is providedon the insulating base 210. In FIGS. 2 and 3, the first ground conductor230 includes a plurality of ground conductors and is disposed in theinsulating base 210. The notch portion 240 is formed on a side surface211 of the insulating base 210 such that the first ground conductor 230is partially exposed. The notch portion 240 is formed with the conductor250 electrically connected to the first ground conductor. The notchportion 240 and the conductor 250 constitute a castellation. The firstelectrode 260 is exposed on a main surface 212 (upper surface in FIG. 2)of the insulating base 210 facing the flexible board 300. The firstelectrode 260 is electrically connected to the first transmission line220. In FIG. 2, a resist 280 is applied to the upper surfaces of theinsulating base 210 and the first transmission line 220. The resist 280has a plurality of openings, and a part of the first transmission line220 exposed from one of the openings is used as the first electrode 260.

As shown in FIGS. 2 and 3, the flexible board 300 includes an insulatingsheet 310, a second transmission line 320, a second ground conductor330, a second electrode 340, and a third electrode 350.

The insulating sheet 310 includes a plurality of insulating layers. Theinsulating sheet 310 is provided with the second transmission line 320that transmits an electrical signal to an optical element provided inthe optical subassembly 100. In FIG. 2, the second transmission line 320is provided between the insulating layer 311 and the insulating layer312. The second ground conductor 330 is disposed in the insulating sheet310, and is provided between the insulating layer 312 and the insulatinglayer 313 in FIG. 3. The second electrode 340 is exposed on at least amain surface 314 of the insulating sheet 310 facing the printed circuitboard 200. The second electrode 340 is connected to the secondtransmission line 320 as shown in FIG. 2. In FIGS. 2 and 3, the secondelectrode 340 has a configuration in which two electrodes sandwich aninsulating layer 312 and the two electrodes are electrically connectedby solder filling a via 341. The third electrode 350 is exposed on themain surface 314 of the insulating sheet 310. The third electrode 350 isconnected to the second ground conductor 330. In FIG. 3, the insulatinglayer 313 that is the outermost layer included in the insulating sheet310 has an opening, and a part of the second ground conductor 330exposed from the opening is used as the third electrode 350.

FIG. 4 is a perspective view showing a state in which the printedcircuit board 200 and the flexible board 300 according to the presentimplementation are connected, when viewed from above. FIG. 5 is aperspective view showing a state in which the printed circuit board 200and the flexible board 300 according to the present implementation areconnected, when viewed from below.

As shown in FIG. 4, the first electrode 260 (see FIG. 2) electricallyconnected to the first transmission line 220 provided on the printedcircuit board 200 and the second electrode 340 (see FIG. 3) connected tothe second transmission line 320 provided on the flexible board 300 areelectrically connected by solder 70 filling the via 341 provided in thesecond electrode 340. As shown in FIG. 5, the conductor 250 formed inthe notch portion 240 provided in the printed circuit board 200 andelectrically connected to the first ground conductor 230 and the thirdelectrode 350 (see FIG. 3) connected to the second ground conductor 330provided on the flexible board 300 are connected by solder 74 appliedfrom the conductor 250 to the third electrode 350.

The above-mentioned configuration of the printed circuit board 200 andthe flexible board 300 can improve the reliability of ground connectionbetween the printed circuit board 200 and the flexible board 300. Thatis, in the configuration of the present implementation, when theconductor 250 connected to the first ground conductor 230 in the printedcircuit board 200 and the third electrode 350 connected to the secondground conductor 330 of the flexible board 300 are electricallyconnected by the solder 74, the connection portion can be visuallyrecognized from the outside. Therefore, the reliability of groundconnection between the printed circuit board 200 and the flexible board300 can be improved.

Further, in FIG. 2, the printed circuit board 200 includes two fourthelectrodes 270 connected to the first ground conductor 230. In FIG. 2,the resist 280 has a plurality of openings, and a part of the firstground conductor 230 exposed from two of the openings is used as thefourth electrode 270. That is, the resist 280 formed on the uppersurface of the insulating base 210 of the printed circuit board 200 isnot applied to the first electrode 260 and the fourth electrode 270, andthe first electrode 260 and the fourth electrode 270 are exposed fromthe resist 280. Due to the presence of the resist 280, the solderapplied to the first electrode 260 and the fourth electrode 270 can beprevented from flowing in the direction of the main surface 212 of theinsulating base 210.

In FIGS. 2 and 3, the flexible board 300 includes two fifth electrodes360 connected to the second ground conductor 330. The fifth electrode360 has a configuration in which two electrodes sandwich the insulatinglayer 312 and the two electrodes are electrically connected by solderfilling a via 361.

As shown in FIG. 2, the fourth electrode 270 provided on the printedcircuit board 200 and the fifth electrode 360 provided on the flexibleboard 300 are electrically connected by solder 72 filling the via 361provided in the fifth electrode 360 as shown in FIG. 4. Such aconfiguration can further strengthen the ground connection between theprinted circuit board 200 and the flexible board 300.

An optical module is enhanced in speed and reduced in size and cost withthe spread of broadband networks in recent years. In order to reduce thesize of the optical module 1, not only the optical subassembly 100included in the optical module 1 but also the IC and the printed circuitboard 200 are required to be reduced. In order to reduce the size of theIC, recent IC packages are offered as ball grid array (BGA) types, andthe ball-shaped terminals at that time are arranged at a pitch ofapproximately 0.5 mm. Furthermore, high-density mounting is realized byadopting a full grid configuration in which all of grids on a terminalmounting surface are filled with terminals.

However, since it is difficult to pass the wiring between terminals of0.5 mm, among the terminals disposed in a grid pattern, it is necessaryto connect the printed circuit board 200 to the terminals disposedinside the second and subsequent rows of the grid by using wiringextending between a plurality of insulating layers constituting theinsulating base 210 of the printed circuit board 200.

Here, since the land diameter increases and interference occurs betweenadjacent terminals in the case of forming a via in the IC with a drill,it is difficult to wire the inner terminals of the second and subsequentrows in the full grid. Therefore, it is desirable to form a via by usinga laser in order to reduce the land diameter. All the terminals of thefull grid provided in the IC can be connected to the wiring extendingbetween the plurality of insulating layers in the printed circuit board200. Therefore, the material of the insulating base 210 constituting theprinted circuit board 200 receives restrictions such as laser output andcost, and thus the base thickness, the dielectric constant, thedielectric loss tangent, and the like are naturally determined. Theprinted circuit board 200 includes the first ground conductor 230composed of a plurality of ground conductors in order to enablehigh-density mounting. As shown in FIG. 5, the plurality of groundconductors included in the first ground conductor 230 are electricallyconnected to each other by a plurality of vias 290 that penetrate theprinted circuit board 200.

In recent years, a demand for optical modules capable of transmittinghigh-speed electrical signals of 50 Gbit/s has been increasing due tohigh speed requirements. Under such high-speed transmission, in the caseof connecting the printed circuit board 200 including the first groundconductor 230 composed of a plurality of ground conductors to theoptical subassembly 100 by using the solder or the like via the flexibleboard 300, the ground resonance at the connection point between theprinted circuit board 200 and the flexible board 300 becomes large, andthe waveform quality of the optical module 1 may be deteriorated. Thatis, at the connection portion between the printed circuit board 200 andthe flexible board 300, the signal wiring of the flexible board 300 mayserve as an excitation source and induce parallel plate resonance.

However, as shown in FIGS. 4 and 5, in the optical module 1 according tothe present implementation, the first ground conductor 230 of theprinted circuit board 200 and the second ground conductor 330 of theflexible board 300 are connected by the shortest path by using thesolder 74, and therefore the delay of the return current can besuppressed and the occurrence of the parallel plate resonance phenomenoncan be suppressed.

FIG. 6 is a graph obtained by calculating the transmissioncharacteristics from the printed circuit board 200 to the flexible board300 by using a high frequency structure simulator (HFSS) as athree-dimensional electromagnetic field simulator, according to anexample and a comparative example of the present implementation. Thegraph of the example shown in FIG. 6 shows the transmissioncharacteristics in a state where the printed circuit board 200 includesthe notch portion 240 and the conductor 250, and the conductor 250 andthe third electrode 350 (see FIG. 3) of the flexible board 300 areconnected by the solder 74 as shown in FIGS. 4 and 5. The graph of thecomparative example shown in FIG. 6 shows the transmissioncharacteristics in a state where the printed circuit board 200 does notinclude the conductor 250. That is, the connection between the groundconductors of the printed circuit board 200 and the flexible board 300in the comparative example is only the connection between the fourthelectrode 270 of the printed circuit board 200 and the fifth electrode360 of the flexible board 300.

As shown in FIG. 6, compared with the graph of the comparative example,in the graph of the example, since the first ground conductor 230 of theprinted circuit board 200 and the second ground conductor 330 of theflexible board 300 are connected by the shortest path by using thesolder 74, it is understood that the deterioration of the transmissioncharacteristics due to the parallel plate resonance phenomenon issuppressed.

As long as the occurrence of this parallel plate resonance phenomenon isto be suppressed, a through-hole penetrating the printed circuit board200 may be provided, and the first ground conductor 230 and the secondground conductor 330 may be connected via the through-hole. In contrast,in the present implementation, as described above, the notch portion 240is provided in the side surface 211 of the printed circuit board 200,and the conductor 250 connected to the first ground conductor is formedin the notch portion 240. With such a configuration, when the conductor250 connected to the first ground conductor 230 and the third electrode350 connected to the second ground conductor 330 are electricallyconnected by the solder 74, the connection portion can be visuallyrecognized from the outside. As a result, the reliability of groundconnection between the printed circuit board 200 and the flexible board300 can be improved. As shown in FIG. 1, the flexible board 300 is oftenstored in the optical module 1 in a bent state. Therefore, the flexibleboard 300 may be peeled off from the printed circuit board 200, forexample, upward in FIG. 1. Since the solder 72 remains only in theexposed region of each electrode at the connection point between thefourth electrode 270 of the printed circuit board 200 and the fifthelectrode 360 of the flexible board 300, a sufficient amount of soldermay not be supplied and sufficient connection strength may not beobtained. However, since the solder 74 can be supplied to a region wideto some extent by connecting the conductor 250 connected to the firstground conductor 230 and the third electrode 350 connected to the secondground conductor 330 by the solder 74 provided in the notch portion 240,sufficient connection strength can be obtained, and an effect ofsuppressing the flexible board 300 from being peeled off from theprinted circuit board 200 can be obtained. That is, not only theconnection reliability at the time of manufacturing but also theconnection reliability from a long-term viewpoint can be ensured.Furthermore, rather than simply connecting to the ground at the side ofthe printed circuit board 200, the notch 240 is provided and the groundconnection is made thereat, whereby the effect that the solder 74remains in the vicinity of the connection point can be obtained. In acase where the notch portion 240 is not provided, that is, in a casewhere the conductor is provided on the entire side of the printedcircuit board 200, since the solder spreads over the entire side, thesolder may not remain at the connection point between the printedcircuit board 200 and the flexible board 300, the solder may beinsufficient at the connection portion, and the necessary connectionstrength may not be obtained. This problem may be solved by providingthe notch portion 240. It is possible to secure a component mountingregion on the printed circuit board 200 by providing the notch portion240 and the conductor 250 rather than forming a through-hole penetratingthe printed circuit board 200. Therefore, according to the configurationof the present implementation, it is possible to strengthen the groundconnection between the printed circuit board 200 and the flexible board300 while expanding the component mounting region in the printed circuitboard 200, and it is also possible to improve the visibility of theground connection portion of the printed circuit board 200 and theflexible board 300.

FIG. 7 is a schematic view showing a cross-sectional structure of thefirst transmission line 220 and the center line of the secondtransmission line 320. As shown in FIG. 7, since the conductor 250provided on the side surface 211 of the printed circuit board 200, andthe third electrode 350 of the flexible board 300 are orthogonal to eachother, fillets are easily formed, and the strength of the connectionportion between the printed circuit board 200 and the flexible board 300can be improved.

FIG. 8 is a schematic perspective view showing a cross-sectionalstructure of the first transmission line 220 and the center line of thesecond transmission line 320. As shown in FIGS. 7 and 8, the opticalmodule 1 according to the present implementation is configured such thatthe first ground conductor 230 has an opening 230A at a positionoverlapping the first electrode 260 when viewed from the directionorthogonal to the main surface 212 of the insulating base 210. Such aconfiguration can further improve the accuracy of impedance matching.For example, even in a case where displacement occurs between the firstelectrode 260 of the printed circuit board 200 and the second electrode340 of the flexible board 300 in the manufacturing process, in order toreduce the change in the contact area between the first electrode 260and the second electrode 340, the width (length in the directionperpendicular to the extending direction of the first transmission line220) of the first electrode 260 is made larger than the width of thefirst transmission line 220 as shown in FIG. 2. For this reason, theimpedance decreases as the area of the first electrode 260 increases.With respect to this problem, as shown in FIGS. 7 and 8, the firstground conductor 230 has the opening 230A at a position overlapping thefirst electrode 260 when viewed from the direction orthogonal to themain surface 212 of the insulating base 210, thereby making it possibleto adjust impedance and perform impedance matching.

FIGS. 9, 10, and 11 are schematic plan views of the printed circuitboard 200 according to the present implementation. FIG. 12 is a graphobtained by calculating the transmission characteristics from theprinted circuit board 200 to the flexible board 300 for theconfigurations according to FIGS. 9, 10, and 11 by using a highfrequency structure simulator (HFSS) as a three-dimensionalelectromagnetic field simulator.

In the configuration shown in FIG. 9, a center line 260A of the firstelectrode 260 overlaps the notch portion 240 when viewed from thedirection orthogonal to the main surface 212 of the insulating base 210.In the configuration shown in FIG. 9, a center line 240A of the notchportion 240 and the center line 260A of the first electrode 260 coincidewith each other when viewed from the direction orthogonal to the mainsurface 212 of the insulating base 210. Here, the center line 260A ofthe first electrode 260 means a straight line parallel to the extendingdirection of the first transmission line 220 and passing through thecenter position of the first electrode 260 in the direction orthogonalto the extending direction. The center line 240A of the notch portion240 means a straight line parallel to the extending direction of thefirst transmission line 220 and passing through the center position ofthe notch portion 240 in the direction orthogonal to the extendingdirection.

In the configuration shown in FIG. 10, the center line 260A of the firstelectrode 260 overlaps the notch portion 240 when viewed from thedirection orthogonal to the main surface 212 of the insulating base 210.In the configuration shown in FIG. 10, the center line 240A of the notchportion 240 and the center line 260A of the first electrode 260 do notcoincide with each other when viewed from the direction orthogonal tothe main surface 212 of the insulating base 210.

In the configuration shown in FIG. 11, the center line 260A of the firstelectrode 260 does not overlap the notch portion 240 when viewed fromthe direction orthogonal to the main surface 212 of the insulating base210. In the configuration shown in FIG. 11, the center line 240A of thenotch portion 240 and the center line 260A of the first electrode 260 donot coincide with each other when viewed from the direction orthogonalto the main surface 212 of the insulating base 210.

In FIG. 12, the transmission characteristics for the configuration shownin FIG. 9 are displayed as “center line coinciding”. The transmissioncharacteristics for the configuration shown in FIG. 10 are displayed as“center line not coinciding”. The transmission characteristics for theconfiguration shown in FIG. 11 are displayed as “not overlapping”.

As shown in FIG. 12, it is understood that in the transmissioncharacteristics according to the configurations shown in FIGS. 9 and 10,dip is less and stable transmission can be performed, compared with thetransmission characteristics according to the configuration shown inFIG. 11. In particular, in order to perform transmission of 50 Gbit/s,it is necessary to perform stable transmission up to 35 GHz. Theconfigurations shown in FIGS. 9 and 10 are desirable because thetransmission characteristics do not have a large dip even in thevicinity of 35 GHz. Accordingly, as shown in FIGS. 9 and 10, it isdesirable that the center line 260A of the first electrode 260 overlapsthe notch portion 240 when viewed from the direction orthogonal to themain surface 212 of the insulating base 210.

As shown in FIG. 12, comparing the transmission characteristicsaccording to the configuration shown in FIG. 9 and the transmissioncharacteristics according to the configuration shown in FIG. 10, it isunderstood that in the configuration shown in FIG. 9 in which the centerline 240A of the notch portion 240 coincides with the center line 260Aof the first electrode 260 when viewed from the direction orthogonal tothe main surface 212 of the insulating base 210, there is no large dipeven in the transmission over 40 GHz and stable transmission can beperformed. Therefore, from the viewpoint of stable signal transmission,it is desirable that the center line 240A of the notch portion 240 andthe center line 260A of the first electrode 260 coincide with each otheras shown in FIG. 9.

On the other hand, with the configuration shown in FIG. 10, since theallowable range for the position where the notch portion 240 is disposedincreases, the degree of freedom of component mounting on the printedcircuit board 200 increases. Therefore, it can be said that theconfiguration shown in FIG. 10 is more desirable than the configurationshown in FIG. 9 from the viewpoint of the degree of design freedomregarding component mounting on the printed circuit board 200. That is,the configuration shown in FIG. 10 can be said to be a configurationthat realizes both the stability of transmission and the degree offreedom of component mounting.

FIG. 13 is a schematic plan view in which the overlapping portion of thenotch portion 240 and the third electrode 350 according to the presentimplementation is enlarged. As shown in FIG. 13, in the extendingdirection of the insulating sheet 310, the entire third electrode 350 isdisposed at the same position on the printed circuit board 200 as theside surface 211 where the notch portion 240 is formed or on an innerside of the printed circuit board 200 further than the side surface 211.In FIG. 13, the entire third electrode 350 is disposed on the inner sideof the printed circuit board 200 further than the side surface 211 inthe extending direction of the insulating sheet 310. By adopting such aconfiguration, the following advantages can be obtained.

First, the flexible board 300 has a role of absorbing variations in theoverall length of the optical module 1. The flexible board 300 has arole of absorbing stress generated on the printed circuit board 200 sideand suppressing transmission of the stress to the optical subassembly100. In the present implementation, as shown in FIGS. 5 and 7, byadopting a configuration in which the entire third electrode 350 isdisposed at the same position as the side surface 211 or on the innerside of the printed circuit board 200 further than the side surface 211in the extending direction of the insulating sheet 310, due to thepresence of the solder 74, the influence of the loss of flexibility inthe extending direction of the insulating sheet 310 on the flexibleboard 300 can be reduced and the loss of the two roles described abovecan be suppressed.

The optical module 1 according to the present implementation asdescribed above can be manufactured through the following manufacturingprocess.

An in-process printed circuit board including the insulating base 210,the first transmission line 220 provided on the insulating base 210, thefirst ground conductor 230 disposed in the insulating base 210, and thefirst electrode 260 exposed on the main surface 212 of the insulatingbase 210 facing the flexible board 300 and electrically connected to thefirst transmission line 220 is prepared.

Next, as shown in FIGS. 2 and 3, the notch portion 240 that exposes apart of the first ground conductor 230 from the side surface 211 of theinsulating base 210 is formed.

Thereafter, the conductor 250 that is electrically connected to thefirst ground conductor 230 is formed at the notch portion 240. In a casewhere the first ground conductor 230 includes a plurality of groundconductors, the plurality of ground conductors are electricallyconnected to each other by forming the conductor 250.

A process for preparing the flexible board 300 is performed in parallelwith or before and after the three processes of a process of preparingthe in-process printed circuit board described above, a process offorming the notch portion 240, and a process of forming conductor 250.That is, the order of the above three processes and the process ofpreparing the flexible board 300 is not limited. In the process ofpreparing the flexible board 300, as shown in FIGS. 2 and 3, theflexible board 300 including the insulating sheet 310 including aplurality of insulating layers 311, 312, and 313, the secondtransmission line 320 provided on the insulating sheet 310, the secondground conductor 330 disposed in the insulating sheet 310, the secondelectrode 340 exposed on the main surface 314 of the insulating sheet310 facing the printed circuit board 200 and connected to the secondtransmission line 320, and the third electrode 350 exposed on the mainsurface 314 of the insulating sheet 310 and connected to the secondground conductor 330 is prepared.

Thereafter, as shown in FIG. 5, the upper surface of the printed circuitboard 200 is mounted on the back-surface side of the flexible board 300.At that time, when viewed from the direction orthogonal to the mainsurface 212 of the printed circuit board 200, the flexible board 300 andthe printed circuit board 200 are disposed so that at least a part ofthe third electrode 350 electrically connected to the second groundconductor 330 in the flexible board 300 overlaps at least a part of aregion surrounded by the conductor 250 and the side surface 211.

As shown in FIG. 5, the solder 74 is applied from the conductor 250 ofthe printed circuit board 200 to the third electrode 350 of the flexibleboard 300, and the conductor 250 and the third electrode 350 areelectrically connected.

By such a manufacturing process, the optical module 1 according to thepresent implementation described above can be manufactured.

It is possible to suppress a change in impedance caused by groundconnection between the printed circuit board 200 and the flexible board300 by using such a manufacturing method. That is, when the printedcircuit board 200 and the flexible board 300 are connected to theground, for example, in a case where heat is applied from the insulatinglayer 311 (see FIG. 4) side of the flexible board 300 in order to meltthe solder disposed between the two, a part of the insulating layer 311may be peeled off due to the heat. When a part of the insulating layer311 is peeled off, voids may be generated in the insulating layer 311,the dielectric constant in the insulating layer 311 may change, and theimpedance may change. However, with the above manufacturing method, theconductor 250 and the third electrode 350 can be electrically connectedby the solder 74 without applying such heat from the insulating layer311 side. As a result, a change in impedance caused by ground connectionbetween the printed circuit board 200 and the flexible board 300 can besuppressed.

The foregoing disclosure provides illustration and description, but isnot intended to be exhaustive or to limit the implementations to theprecise forms disclosed. Modifications and variations may be made inlight of the above disclosure or may be acquired from practice of theimplementations.

Even though particular combinations of features are recited in theclaims and/or disclosed in the specification, these combinations are notintended to limit the disclosure of various implementations. In fact,many of these features may be combined in ways not specifically recitedin the claims and/or disclosed in the specification. Although eachdependent claim listed below may directly depend on only one claim, thedisclosure of various implementations includes each dependent claim incombination with every other claim in the claim set.

No element, act, or instruction used herein should be construed ascritical or essential unless explicitly described as such. Also, as usedherein, the articles “a” and “an” are intended to include one or moreitems, and may be used interchangeably with “one or more.” Further, asused herein, the article “the” is intended to include one or more itemsreferenced in connection with the article “the” and may be usedinterchangeably with “the one or more.” Furthermore, as used herein, theterm “set” is intended to include one or more items (e.g., relateditems, unrelated items, a combination of related and unrelated items,etc.), and may be used interchangeably with “one or more.” Where onlyone item is intended, the phrase “only one” or similar language is used.Also, as used herein, the terms “has,” “have,” “having,” or the like areintended to be open-ended terms. Further, the phrase “based on” isintended to mean “based, at least in part, on” unless explicitly statedotherwise. Also, as used herein, the term “or” is intended to beinclusive when used in a series and may be used interchangeably with“and/or,” unless explicitly stated otherwise (e.g., if used incombination with “either” or “only one of”).

What is claimed is:
 1. An optical module comprising: a printed circuitboard; and a flexible board, wherein the printed circuit board includes:an insulating base, a first transmission line provided on the insulatingbase, a first ground conductor disposed in the insulating base, a notchportion formed on a side surface of the insulating base such that thefirst ground conductor is partially exposed, a conductor provided in thenotch portion and electrically connected to the first ground conductor,and a first electrode exposed on a main surface of the insulating basefacing the flexible board and electrically connected to the firsttransmission line, wherein the flexible board includes: an insulatingsheet including a plurality of insulating layers, a second transmissionline provided on the insulating sheet, a second ground conductordisposed in the insulating sheet, a second electrode exposed on a mainsurface of the insulating sheet facing the printed circuit board andconnected to the second transmission line, and a third electrode exposedon the main surface of the insulating sheet facing the printed circuitboard and connected to the second ground conductor, wherein the firstelectrode and the second electrode are electrically connected, andwherein the conductor and the third electrode are connected by solder.2. The optical module of claim 1, wherein the notch portion and theconductor constitute a castellation.
 3. The optical module of claim 1,wherein the first ground conductor is composed of a plurality of groundconductors.
 4. The optical module of claim 1, wherein the thirdelectrode is a part of the second ground conductor exposed from theinsulating sheet.
 5. The optical module of claim 1, wherein a centerline of the first electrode overlaps the notch portion when viewed froma direction orthogonal to the main surface of the insulating base. 6.The optical module of claim 5, wherein a center line of the notchportion and the center line of the first electrode do not coincide witheach other when viewed from the direction orthogonal to the main surfaceof the insulating base.
 7. The optical module of claim 5, wherein acenter line of the notch portion and the center line of the firstelectrode coincide with each other when viewed from the directionorthogonal to the main surface of the insulating base.
 8. The opticalmodule of claim 1, wherein the entire third electrode is disposed at asame position as the side surface or on an inner side of the printedcircuit board further than the side surface in an extending direction ofthe insulating sheet.
 9. The optical module of claim 1, wherein thefirst ground conductor has an opening at a position overlapping thefirst electrode when viewed from the direction orthogonal to the mainsurface of the insulating base.
 10. The optical module of claim 1,wherein the conductor and the third electrode are orthogonal to eachother, and the solder forms a fillet.
 11. A method for manufacturing anoptical module comprising: preparing an in-process printed circuit boardincluding: an insulating base, a first transmission line provided on theinsulating base, a first ground conductor disposed in the insulatingbase, and a first electrode exposed on a main surface of the insulatingbase facing the flexible board and electrically connected to the firsttransmission line; forming a notch portion on a side surface of theinsulating base such that the first ground conductor is partiallyexposed from the side surface; forming a conductor electricallyconnected to the first ground conductor at the notch portion; preparinga flexible board including: an insulating sheet including a plurality ofinsulating layers, a second transmission line provided on the insulatingsheet, a second ground conductor disposed in the insulating sheet, asecond electrode exposed on a main surface of the insulating sheetfacing the printed circuit board and connected to the secondtransmission line, and a third electrode exposed on the main surface ofthe insulating sheet facing the printed circuit board and connected tothe second ground conductor; disposing the flexible board and theprinted circuit board so that at least a part of the third electrodeoverlaps at least a part of a region surrounded by the conductor and theside surface when viewed from a direction orthogonal to the main surfaceof the insulating base; and applying solder from the conductor to theelectrode.
 12. The method of claim 11, wherein the notch portion and theconductor constitute a castellation.
 13. The method of claim 11, whereinthe first ground conductor is composed of a plurality of groundconductors.
 14. The method of claim 11, wherein the third electrode is apart of the second ground conductor exposed from the insulating sheet.15. The method of claim 11, wherein a center line of the first electrodeoverlaps the notch portion when viewed from the direction orthogonal tothe main surface of the insulating base.
 16. The method of claim 11,wherein a center line of the notch portion and a center line of thefirst electrode do not coincide with each other when viewed from thedirection orthogonal to the main surface of the insulating base.
 17. Themethod of claim 11, wherein a center line of the notch portion and acenter line of the first electrode coincide with each other when viewedfrom the direction orthogonal to the main surface of the insulatingbase.
 18. The method of claim 11, wherein the entire third electrode isdisposed at a same position as the side surface or on an inner side ofthe printed circuit board further than the side surface in an extendingdirection of the insulating sheet.
 19. The method of claim 11, whereinthe first ground conductor has an opening at a position overlapping thefirst electrode when viewed from the direction orthogonal to the mainsurface of the insulating base.
 20. The method of claim 11, wherein theconductor and the third electrode are orthogonal to each other, and thesolder forms a fillet.